It's typically either that you have recorded a voicemail greeting in the spot for introductory greeting, or your status is on Do Not Disturb.  Many people call me in IT saying their calls are going straight to voicemail when in reality that greeting plays and then the phone rings.  Call the number and wait for the greeting to end and if it starts ringing afterward, that's your issue.  If not then DND is on somewhere, either on a softphone or on a desk phone.

---